Method of protecting carbon or graphite electrodes in fused electrolytes



No Drawing. Application filed February 23, 1927.

Patented Dec. 20, 1927.

UNITED STATES PATENT oF rcE.

EDGAR ARTHUR ASHCROFT, OF WAYE, NEAR ASHBURTON, ENGLAND, A SSIGNOR TO NEW METALLURGY LIMITED, OF LONDON, ENGLAND, AN INCORPORATED ENGLISH JOINT-STOCK COMPANY.

METHOD OF PROTECTING CARBON OR GRAPHITE ELECTRODES IN FUSED ELECTRO- LYTES.

The present invention relates to improvements in the operation of electrolytic cells employing fused electrolytes, and refers more particularly to the protection of the carbon or graphite electrodes 'from the action of moisture and/or oxygen which normally cause destruction of the said electrodes.

According to the present invention, in an electrolytic process using a fused electrolyte and wherein one or more carbon or graphite electrodes are employed, I provide amethod of protecting said 7 electrodes against attack by oxygen, said method consisting in the introduction of sulphur or a sulphur compound or mixture into the electrolyte or onto the surface thereof in quantity sufficient to ensure the combination with sulphur of the whole of. the oxygen present in the electrolyzer. In this manner the destruction of the electrodes by oxidation is prevented, the protection extending both to the parts of the electrodes immersed in the electrolyte andin the" gas space above the electrolyte. Normally the presence of air in this space burns away the carbon or graphite electrodes due to the high operating temperature of the electrolyzer. On the addition of sulphur, however, the latter burns first by reason of lower ignition temperature. In the electrolyte any oxygen compound which may be present is decomposed -and the oxygen instead of attacking the electrodes combines with the sulphur.

The invention is applicable to electrolytic processes in general in which the fused salts of metals are electrolyzed for the recovery of the metal, and in which normally little or no sulphur or incompletely oxidized'sulphur compounds are present.

It has, however, an especially valuable commercial application in the electrolytic recovery of zinc and lead from fused baths of their chlorides.

In this case the action of the sulphur in be to form protecting the plates appears to S0 01 and various compounds such as S0,,

S(OH) 01 none of which oxidizes the graphite plates at the temperature working. of the elect-rolyzer, whereas oxygen compounds Serial No. 170,412, and in Great Britain November 27, 1925.

which are decomposed yielding oxygen at the anode in the absence of sulphur are highly destructive.

Instead of introducingsulphur I ma introduce sulphur chloride to the electrolyzer in liquid form for a similar purpose or I may introduce any suitable sulphide-such as lead sulphidewhen the electrolyzer is producing leador zinc sulphide-when the electrolyzer is producing zinc. The sulphide will then. be' decomposed the mgltlall the going to the cathode and the resultin phur acting as above described wit anode product.

I may vary the above described process by introducing small quantities of sulphates in conj unctioh with sulphides or sulphur or of sulphites or even of oxidesprovided an excess'of sulphur is present also-the necessary conditions being that the total amount of sulphur present shall always be sufiicient to prevent the liberation of free oxygen at the anode surfaces, and also that nothing but one or other of the above described compounds or analogous compounds which do not oxidize or act odes will be formed. It is then found that there is no attack on the graphite plates even if moist air be introduced to the electrolyzer. Without such precautions as above described the introduction of moist air to an electrodetrimentally on the anlyzer brings about destruction of the plates.

ite electrode is employed, a method of 'protecting said electrode against attack by oxy-' gen consisting in the introduction of sulphur-to the electrolyzer in quantity approximately only suflicient to ensure the combination with sulphur of the whole of the oxygen present in the electrolyzer.

2. In an electrolytic process using a fused electrolyte and wherein a carbon or graphite electrode is employed, a method of protecting said electrode against attack by, oxygen, consisting in the introduction of :1 sulphur compound to the electrolyzer in quantity approximately only sufiicientto ensure the combination with sulphur of the whole of the oxygen present in the electrolyzer.

3. In an electrolytic process using a fused electrolyte and wherein a carbon or graphite electrode is employed, a method of protecting said electrode against attack by oxygen, consisting in the-introduction of metallic sulphides in the electrolyzer in quantity approximately only sufficient to ensurethe combination with sulphur of the whole of the oxygen present in the: electrolyzer.

- 4. In an electrolytic process using a fused electrolyte and wherein a carbon or graphite electrode is employed, a method of proi tecting said electrode against attack by oxygen, consisting in the introduction of the sulphide of the metal which is being recovered to the electrolyzer in quantity approximately only sufiie'ient to ensure the combination with sulphur of the Whole of the oxygen present in the electrolyzer.

EDGAR ARTHUR ASHCROFT. 

